The present invention relates to a solenoid drive circuit for driving a solenoid valve and the like.
FIG. 1 is a prior art solenoid drive circuit for driving a solenoid valve. A solenoid 1 of the solenoid valve is connected to a power source at a first end, and is grounded via a collecter-emitter circuit of a drive transistor 2 at a second end. A serial circuit of a diode 3 and a Zener diode 4 is inserted in parallel with the solenoid I. A base of the drive transistor 2 is connected via a resistor to an input terminal IN wherein a drive signal for driving the solenoid valve is applied to the input terminal IN.
According to the drive circuit of FIG. 1, an off-responsibility of current flowing through the solenoid 1 rises by existence of the Zener diode 4. However, in the drive circuit of FIG. 1, due to existence of the Zener diode 4, current due to a counter-electromotive force cannot be used as a holding current when a holding current lower than a starting current is to be applied to the solenoid 1 via lowering an average current flowing through the solenoid 1 by turning the drive transistor 2 on and off periodically. Because of this, in the configuration of the circuit of FIG. 1, it is necessary to increase the supply of electric power in order to counteract the shortage due to the inability to utilize the current by counter-electromotive force as the holding current, by extending the on time of the drive transistor 2. That is, as shown in the drawing of FIG. 2, which is explanatory of the operation of the circuit of FIG. 1, current which flows through the solenoid 1, when the drive transistor 2 is changed from on to off, cannot change instantaneously as shown by the full line because of the use of the Zener diode. On the other hand, if the Zener diode 4 does not exist, current by counter-electromotive force flows through the solenoid 1 when the drive transistor 2 is changed from on to off, as shown by the broken line. Therefore, if it is possible to utilize the current by counter-electromotive force as the holding current, it is possible to conserve the supply of electric power during a holding period.